fix(rewrite): freeze operands against any later side effect

changelog/14445.bugfix.rst

@@ -0,0 +1,3 @@
+Fixed assertion rewriting evaluating walrus operator (``:=``) expressions multiple times, causing incorrect test results when the expression had side effects (e.g., incrementing a counter or calling a function).
+
+Operands preceding a walrus operator are now evaluated -- and reported -- before it rebinds their name, so ``assert value != identity(value := value.lower())`` keeps Python's left-to-right evaluation order.

changelog/14813.contrib.rst

@@ -0,0 +1 @@
+Added a systematic coverage matrix for assertion rewriting.

changelog/14814.bugfix.rst

@@ -0,0 +1 @@
+Fixed assertion rewriting evaluating a walrus operator (``:=``) or a starred argument out of order when a later argument assigned to the same name, so ``assert collect(*items, identity(items := [9]))`` now passes the pre-assignment ``items``.

changelog/14815.improvement.rst

@@ -0,0 +1,4 @@
+Assertion failure messages now decompose subscript expressions, showing the container and the key that produced a value::
+
+    assert 1 == 99
+     +  where 1 = {'a': 1, 'b': 2}['a']

changelog/14816.improvement.rst

@@ -0,0 +1,4 @@
+Assertion failure messages now show the condition of a conditional expression::
+
+    assert 0 == 99
+     +  where 0 = (... if True else ...)

changelog/14817.improvement.rst

@@ -0,0 +1,4 @@
+Assertion failure messages now show a method call on a single line, without the bound method as a separate intermediate::
+
+    assert 42 == 100
+     +  where 42 = Obj().compute()

changelog/14821.bugfix.rst

@@ -0,0 +1 @@
+Fixed assertion rewriting reading an operand after a later call had rebound its name, so ``assert count == bump()`` compares the ``count`` Python would have compared even when ``bump()`` assigns to it via ``global`` or ``nonlocal``.

scripts/diff-assert-rewrite.py

@@ -0,0 +1,202 @@
+"""Compare assert-rewrite output between two pytest versions (or worktree).
+
+Runs the dump script for both sides (in parallel) and shows a unified diff.
+Supports all three output formats: source, ast, and compact.
+
+Usage::
+
+    # Two released versions:
+    python scripts/diff-assert-rewrite.py --left 7.4.0 --right 8.0.0 example.py
+
+    # Release vs local worktree:
+    python scripts/diff-assert-rewrite.py --left 8.3.0 --right worktree example.py
+
+    # Compact AST diff (strips position noise):
+    python scripts/diff-assert-rewrite.py --left 7.4.0 --right worktree -f compact example.py
+
+    # All formats at once:
+    python scripts/diff-assert-rewrite.py --left 7.4.0 --right 8.0.0 -f all example.py
+"""
+
+from __future__ import annotations
+
+import argparse
+from concurrent.futures import ThreadPoolExecutor
+import difflib
+from pathlib import Path
+import sys
+
+
+_DUMP_SCRIPT = Path(__file__).resolve().parent / "dump-assert-rewrite.py"
+
+_FORMATS = ("source", "ast", "compact")
+
+
+def _label(spec: str) -> str:
+    return "worktree" if spec == "worktree" else f"pytest=={spec}"
+
+
+def get_dump(spec: str, file_path: Path, fmt: str) -> str:
+    """Run the dump script for a single side and return its output."""
+    # Import inline so this file stays lightweight at module level.
+    import subprocess
+
+    args = [sys.executable, str(_DUMP_SCRIPT)]
+    if spec == "worktree":
+        args.append("--worktree")
+    else:
+        args.extend(["--pytest-version", spec])
+    args.extend(["--format", fmt, str(file_path)])
+
+    result = subprocess.run(args, capture_output=True, text=True, check=False)
+    if result.returncode != 0:
+        sys.stderr.write(result.stderr)
+        raise SystemExit(f"Dump failed for {_label(spec)}")
+    return result.stdout
+
+
+def colored_diff(lines: list[str]) -> str:
+    """Apply ANSI colours to a unified-diff line list."""
+    RED = "\033[31m"
+    GREEN = "\033[32m"
+    CYAN = "\033[36m"
+    RESET = "\033[0m"
+
+    out: list[str] = []
+    for line in lines:
+        if line.startswith(("---", "+++")):
+            out.append(f"{CYAN}{line}{RESET}")
+        elif line.startswith("-"):
+            out.append(f"{RED}{line}{RESET}")
+        elif line.startswith("+"):
+            out.append(f"{GREEN}{line}{RESET}")
+        elif line.startswith("@@"):
+            out.append(f"{CYAN}{line}{RESET}")
+        else:
+            out.append(line)
+    return "\n".join(out)
+
+
+def show_diff(
+    left_text: str,
+    right_text: str,
+    *,
+    left_label: str,
+    right_label: str,
+    fmt: str,
+    context: int,
+    use_color: bool,
+) -> bool:
+    """Print a unified diff; return True if differences were found."""
+    if left_text == right_text:
+        return False
+
+    diff_lines = list(
+        difflib.unified_diff(
+            left_text.splitlines(),
+            right_text.splitlines(),
+            fromfile=f"{left_label} [{fmt}]",
+            tofile=f"{right_label} [{fmt}]",
+            n=context,
+        )
+    )
+
+    if use_color:
+        print(colored_diff(diff_lines))
+    else:
+        print("\n".join(diff_lines))
+
+    return True
+
+
+def main(argv: list[str] | None = None) -> None:
+    parser = argparse.ArgumentParser(
+        description=__doc__,
+        formatter_class=argparse.RawDescriptionHelpFormatter,
+    )
+    parser.add_argument(
+        "--left",
+        required=True,
+        metavar="VER|worktree",
+        help="Left side: pytest version or 'worktree'",
+    )
+    parser.add_argument(
+        "--right",
+        required=True,
+        metavar="VER|worktree",
+        help="Right side: pytest version or 'worktree'",
+    )
+    parser.add_argument(
+        "--format",
+        "-f",
+        dest="fmt",
+        choices=(*_FORMATS, "all"),
+        default="source",
+        help="Output format (default: source)",
+    )
+    parser.add_argument(
+        "--context",
+        "-C",
+        type=int,
+        default=3,
+        help="Context lines in diff (default: 3)",
+    )
+    parser.add_argument(
+        "--no-color",
+        action="store_true",
+        help="Disable coloured output",
+    )
+    parser.add_argument("file", type=Path, help="Python file to compare")
+    args = parser.parse_args(argv)
+
+    if not args.file.is_file():
+        raise SystemExit(f"File not found: {args.file}")
+
+    formats = _FORMATS if args.fmt == "all" else (args.fmt,)
+    use_color = not args.no_color and sys.stdout.isatty()
+    left_label = _label(args.left)
+    right_label = _label(args.right)
+
+    # Fetch all needed dumps in parallel (both sides x all formats).
+    jobs: dict[tuple[str, str], str] = {}
+    with ThreadPoolExecutor(max_workers=len(formats) * 2) as pool:
+        futures = {
+            (side, fmt): pool.submit(get_dump, spec, args.file, fmt)
+            for fmt in formats
+            for side, spec in [("left", args.left), ("right", args.right)]
+        }
+        for key, future in futures.items():
+            jobs[key] = future.result()
+
+    any_diff = False
+    for fmt in formats:
+        if len(formats) > 1:
+            header = f"=== {fmt} ==="
+            if use_color:
+                header = f"\033[1m{header}\033[0m"
+            print(header)
+
+        had_diff = show_diff(
+            jobs[("left", fmt)],
+            jobs[("right", fmt)],
+            left_label=left_label,
+            right_label=right_label,
+            fmt=fmt,
+            context=args.context,
+            use_color=use_color,
+        )
+        if not had_diff:
+            print(
+                f"No differences in {fmt} output between {left_label} and {right_label}"
+            )
+        else:
+            any_diff = True
+
+        if len(formats) > 1:
+            print()
+
+    raise SystemExit(1 if any_diff else 0)
+
+
+if __name__ == "__main__":
+    main()

scripts/dump-assert-rewrite.py

@@ -0,0 +1,166 @@
+"""Dump the assert-rewritten form of a Python file for a specific pytest version.
+
+Uses ``uv run`` to execute the rewriter in an ephemeral environment, so any
+released pytest version can be inspected without installing it globally.
+
+Usage::
+
+    # Rewritten source (default):
+    python scripts/dump-assert-rewrite.py --pytest-version 8.0.0 example.py
+
+    # Using local worktree:
+    python scripts/dump-assert-rewrite.py --worktree example.py
+
+    # Compact AST (best for diffing -- no position attributes):
+    python scripts/dump-assert-rewrite.py --worktree --format compact example.py
+
+    # Full AST with positions:
+    python scripts/dump-assert-rewrite.py --pytest-version 7.4.0 --format ast example.py
+"""
+
+from __future__ import annotations
+
+import argparse
+import os
+from pathlib import Path
+import subprocess
+import sys
+import textwrap
+
+
+# Self-contained script executed inside the target pytest environment.
+# Reads source from stdin, writes the rewritten form to stdout.
+_WORKER_SCRIPT = textwrap.dedent("""\
+    import ast
+    import sys
+
+    source = sys.stdin.buffer.read()
+    fmt = sys.argv[1]
+
+    try:
+        from _pytest.assertion.rewrite import rewrite_asserts
+    except ImportError:
+        sys.exit("Could not import rewrite_asserts from this pytest version")
+
+    tree = ast.parse(source)
+    try:
+        rewrite_asserts(tree, source)
+    except TypeError:
+        # pytest < 6 did not accept the source parameter
+        tree = ast.parse(source)
+        rewrite_asserts(tree)
+
+    ast.fix_missing_locations(tree)
+
+    if fmt == "source":
+        print(ast.unparse(tree))
+    elif fmt == "ast":
+        print(ast.dump(tree, indent=2))
+    elif fmt == "compact":
+        print(ast.dump(tree, indent=2, include_attributes=False))
+    else:
+        sys.exit(f"Unknown format: {fmt!r}")
+""")
+
+
+def run_worker(
+    *,
+    pytest_version: str | None,
+    worktree: bool,
+    file_content: bytes,
+    fmt: str,
+) -> str:
+    """Execute the worker script and return its stdout."""
+    if worktree:
+        repo_root = Path(__file__).resolve().parent.parent
+        env = os.environ.copy()
+        env["PYTHONPATH"] = (
+            str(repo_root / "src") + os.pathsep + env.get("PYTHONPATH", "")
+        )
+        cmd = [sys.executable, "-c", _WORKER_SCRIPT, fmt]
+    else:
+        assert pytest_version is not None
+        cmd = [
+            "uv",
+            "run",
+            "--no-project",
+            "--with",
+            f"pytest=={pytest_version}",
+            "--",
+            "python",
+            "-c",
+            _WORKER_SCRIPT,
+            fmt,
+        ]
+        env = None
+
+    try:
+        result = subprocess.run(
+            cmd, input=file_content, capture_output=True, check=False, env=env
+        )
+    except FileNotFoundError as exc:
+        if "uv" in str(exc):
+            raise SystemExit(
+                "'uv' not found — install it: https://docs.astral.sh/uv/"
+            ) from exc
+        raise
+
+    if result.returncode != 0:
+        label = version_label(pytest_version=pytest_version, worktree=worktree)
+        sys.stderr.buffer.write(result.stderr)
+        raise SystemExit(f"Worker failed for {label}")
+
+    return result.stdout.decode()
+
+
+def version_label(*, pytest_version: str | None = None, worktree: bool = False) -> str:
+    """Human-readable label for a pytest source."""
+    return "worktree" if worktree else f"pytest=={pytest_version}"
+
+
+def build_parser() -> argparse.ArgumentParser:
+    parser = argparse.ArgumentParser(
+        description=__doc__,
+        formatter_class=argparse.RawDescriptionHelpFormatter,
+    )
+    src = parser.add_mutually_exclusive_group(required=True)
+    src.add_argument(
+        "--pytest-version",
+        metavar="VER",
+        help="Released pytest version (e.g. 8.0.0)",
+    )
+    src.add_argument(
+        "--worktree",
+        action="store_true",
+        help="Use the local worktree's src/",
+    )
+    parser.add_argument(
+        "--format",
+        dest="fmt",
+        choices=("source", "ast", "compact"),
+        default="source",
+        help="Output format (default: source)",
+    )
+    parser.add_argument("file", type=Path, help="Python file to rewrite")
+    return parser
+
+
+def main(argv: list[str] | None = None) -> None:
+    args = build_parser().parse_args(argv)
+
+    if not args.file.is_file():
+        raise SystemExit(f"File not found: {args.file}")
+
+    output = run_worker(
+        pytest_version=args.pytest_version,
+        worktree=args.worktree,
+        file_content=args.file.read_bytes(),
+        fmt=args.fmt,
+    )
+    sys.stdout.write(output)
+    if output and not output.endswith("\n"):
+        sys.stdout.write("\n")
+
+
+if __name__ == "__main__":
+    main()